WASHINGTON (Reuters) -- A U.S. diplomat fired in Afghanistan has said that the United Nations not only ignored massive fraud in the August election but also told him to keep quiet, then dissembled about his firing.

Peter Galbraith, former deputy to U.N. special envoy Kai Eide, wrote in the Washington Post on October 4 that Secretary-General Ban Ki-moon's final instruction before firing him last week was "Do not talk to the press."

He agreed, then received assurances from an immediate supervisor that his dismissal statement would cite a dispute over how the U.N. mission was handling Afghan electoral fraud.

Instead, he said, U.N. officials announced his firing was in the "best interests of the mission" and leaked that there was personality clash with Eide, his longtime friend.

"I might have tolerated even this last act of dishonesty in a dispute dating back many months if the stakes were not so high," wrote Galbraith, an ally of Richard Holbrooke, the U.S. point man for Afghanistan and Pakistan.

"But in my view, the fraud was a fact that the United Nations had to acknowledge or risk losing its credibility with the many Afghans who did not support President Hamid Karzai," wrote Galbraith, a former U.S. ambassador to Croatia.

The outcome of the August 20 election has yet to be decided, amid accusations of massive fraud, and in public all Western diplomatic missions in Kabul say they are reserving judgment until a complaints process is complete.

Galbraith, son of the late Canadian-American economist John Kenneth Galbraith, chronicled U.N. efforts to ignore fraud allegations for fear of raising tensions in the country.

He said he was told to stop talking about 1,500 polling stations so insecure they could not open on Election Day.

"Eide ordered me not to discuss the ghost polling centers any further," he wrote. "On Election Day, these sites produced hundreds of thousands of phony Karzai votes.

"At other critical stages in the election process, I was similarly ordered not to pursue the issue of fraud."

Those included evidence collected by his staff on hundreds of individual cases of fraud as well as information on southern provinces were more votes were reported than cast, he said.

Galbraith called the elections, which he said were managed by a pro-Karzai election council "a foreseeable train wreck".

He said the fraud "handed the Taliban its greatest strategic victory in eight years of fighting the United States and its Afghan partners".

U.S. officials have cited the dispute over the election results as a main reason for President Barack Obama's decision last month to review its whole policy toward the region, where more than 60,000 American troops are deployed.

"President Obama needs a legitimate Afghan partner to make any new strategy for the country work," Galbraith wrote.

"However, the extensive fraud that took place on Aug. 20 virtually guarantees that a government emerging from the tainted vote will not be credible with many Afghans."